Method and apparatus for controlling high frequency current



Dec. 13, 1955 G. H. DEWITZ 2,727,211

METHOD AND APPARATUS FOR CONTROLLING HIGH FREQUENCY CURRENT Filed Oct. 27, 1951 RADIO )3 LOAD FREQUENCY GENERATOR RADIO 0 5 I M)? FR EQUENOY 26 g 20 GENERATOR O Y /0 T FRE QEZGY O LOAD GENERATOR R.F. SIGNAL CONTROLLED R.F. SIGNAL elgb ard HDewl/iz INVENTOR CONTROL 1 .5 NEY SIGNAL s United States Patent METHOD AND APPARATUS FOR CONTROLLING HIGH FREQUENCY CURRENT Gerhard H. Dewitz, Westport, Conn., assignor to C. G. S. Laboratories, Inc., Stamford, Conn., a corporation of Connecticut Application October 27, 1951, Serial No. 253,541

4 Claims. (Cl. 332-51) This invention relates to methods of and apparatus for controlling the flow of high frequency alternating currents, and more particularly to such methods and apparatus in which the permeability of a conductive element is varied in such manner as to alter the skin effect and thereby change the efiective resistance of the element.

The magnitude of radio frequency current that flows in a circuit may be controlled in any of several ways. For example, the potentials applied to the electrodes of a vacuum tube capable of handling signals of the desired frequency, may be varied so as to cause a corresponding variation in the plate current of the, tube. If the current is made to flow through an inductive winding, the current can be varied by varying the inductance of the winding, for example, by mechanically moving a powdered iron core within the winding. If the current is flowing through a condenser, the amount of current may be controlled by varying the capacity of the condenser. For any particular application, one or another of these methods may be employed, but for many uses and circuit arrangements none of these methods is entirely suitable.

In accordance with the present invention, a simple and easily controlled radio-frequency resistor is provided which offers a minimum of circuit disturbances while at the same time permitting precision control that may be extended over a wide range.

The effective resistance offered by a conductor to the flow of high frequency current is substantially greater than the resistance measured with direct current. When radio frequency current is passed through a conductor, the current tends to crowd to the outside of the couductor so that the center part of the conductor carries less current per unit area than the outer portions of the conductor. This effect is called skin effect and may make the effective resistance of a conductor to the flow of high frequency currents many times the resistance of the same conductor to the flow of lower frequency or direct currents.

For the purposes of this patent, the component of the total resistance that is caused by the skin effect will be termed skin-effect resistance.

This skin eifect causes a re-distribution of the current over the cross-section of the conductor in such manner that most of the current flows where is is enclosed by the smallest number of flux lines, that is, the current distributes itself in such manner as to make the conductor have the least inductance.

This skin effect is present irrespectiive of the shape of the conductor. Thus, with a conductor of circular crosssection the current is concentrated adjacent the outer surface of the conductor; with a flat strip the greatest current density is along the edges of the strip, considerable current flows along the surfaces, and the least current flows through the central portion of the strip.

Skin efiect increases with the permeability of the material through which the current is flowing. Accordingly,

2,727,211 Patented Dec. 13, 1955 ICC it is difficult to cause radio frequency currents to flow through materials having a high permeability because the resistance component introduced by the skin effect.

sidered in conjunction with the accompanying drawings,

in which:

Figure 1 represents diagrammatically a simplified arrangement wherein the radio frequency current is controlled by control of the skin effect of a magnetically permeable conductor;

Figure 2 shows another embodiment of the invention wherein the variable radio frequency resistor is utilized to control a resonant circuit;

Figure 3 shows still another embodiment of the invention in which the magnetic field is produced by a winding around the permeable conductor; and

Figure 4 shows still another embodiment of the invention similar to the resistive element of Figure 3.

As shown in Figure l, a radio frequency generator, indicated diagrammatically at 2, developing a signal of any desired frequenc for example one megacycle, is connected by leads 4 and 6 to a load circuit, indicated diagrammatically at 8. A resistance element 10, formed by a short wire of ferromagnetic material, is connected in series with the lead 6. As used in this example and elsewhere in this application, the term ferromagnetic refers to materials having a magnetic permeability significantly greater than unity. The element 10 may be formed of soft iron, mu metal, silicon steel, magnetic alloys of iron, nickel, and cobalt such as Permalloy, perminvar, permendur, or mu metal, or of other material having a high permeability.

The resistive element It; extends across the gap between the poles of a soft iron core 12 of an electromagnet, generally indicated at 13, which is provided with an energizing winding 14 that is connected through a variable resistance 16 to a battery 18.

The flow of current through the winding 14 produces a magnetic field around the permeable control element 10 and reduces the effective permeability of this wire and accordingly reducing its skin effect resistance. Increasing the current through winding 14 by means of the resistor 16 increases the strength of the magnetic field around the Wire 10 and decreases its skin-effect resistance, permitting a larger radio frequency current to flow from the generator 2 to the load 8.

Thus, by varying the strength of the magnetic field, the flow of current to the load circuit 8 is controlled. It will be apparent that a control of this type is effective for high frequency circuit applications in which other means for controlling the flow of radio frequency current are not well suited.

It will be apparent also that the diagrammatic representation of Figure 1 is intended as indicative of varied configurations. For example, the element 19 can be placed in any position where it is desired to control a radio fre-, quency current, and thus may be used in positions wherenected into a parallel resonant circuit including an inductive winding 20 and a condenser 22. The element is again positioned between the pole faces of the core 12, but in this instance the winding 14 is connected to a source 24 of electrical energy of varying magnitude. For example the source 24 might be a sine Wave generator, or it might be a speech amplifier arranged to impress audio signals on the winding 14, or any other source of control voltage. A bias current source 25 provides for an operating point midway between maximum and minimum skin-effect resistance change produced by the alternating current of source 24.

Radio frequency current is supplied by the generator 2, the output terminals of which are connected to a coil 26 that is coupled to the winding 20. The parallel resonant circuit formed by the inductor and the condenser 22 is resonant at the frequency of the signal delivered by the radio frequency generator 2.

The Q of the resonant circuit varies as a function of the skin-effect resistance of the element 10, and accordingly the Q of the resonant circuit varies in accordance with the amplitude of the signals applied to the winding 14. The radio frequency voltage appearing across the condenser 22, therefore, follows the molulating voltage from the source 24.

Figure 3 shows an arrangement in which a permeable element 10A is the form of a ring formed by a single length of high-permeability iron wire. At one point, this ring 10A is connected by a lead 30 to the radio frequency generator 2. At a point diametrically opposite this connection, the ring 10A is connected to a lead 32 that is connected to any desired load circuit, indicated diagrammatically at 8. A control voltage is provided by a source 34, one terminal of which is connected to one end of a winding 36 that is wound around one half of the ring 10A, the other terminal of this winding being connected through an identical winding 38 to the other terminal of the generator 34. The winding 38 is also wound around the ring or control element 10A and is positioned on the opposite side of the ring from the winding 36. The bias source in series with the source 34 provides a bias current sufiicient to insure operation over a desirable portion of the control characteristic.

The radio frequency current, which travels along the wire 30, divides into two equal components flowing in opposite directions with respect to windings 36 and 38 so that there is no inductive coupling between these windings and the radio frequency current. From the modulating current which fiows through the series windings 36 and 38, a magnetic field is produced which varies the saturation and permeability of the control element 10A. Thus,

if the current through the control windings 36 and 38 is increased, the magnetic saturation of the ring 10A increases, and accordingly reduces its permeability and its skin-effect resistance so that each half of the ring ofiersless skin-etfect resistance to the flow of current from the generator 2 to the load circuit 8.

Figure 4 shows a control element 103, formed of Permalloy or other metal having high magnetic permeability, in the shape of a rectangle with a narrow slit along its center portion so that the winding 36 is wound around one side of the control element 103 and the other winding 38 is wound around the opposite side of the control element.

It is thus seen that the controllable radio frequency resistor provided by the present invention is Well adapted for a large variety of uses in radio frequency devices, and that it is simple and economical to manufacture and may be readily modified and fabricated in many different forms, each adapted for the particular use to which the element is to be put.

I claim:

1. Apparatus for controlling high frequency current comprising a first source of high frequency current, a-

conductive control element having input and output terminals and first and second separate conductive paths conmeeting said input and output terminals, said element being coupled to said first source and arranged to carry high frequency current between said input and output terminals, a second source of current, a first inductive winding surrounding at least a portion of said first conductive path of said element, and a second inductive winding surrounding at least a portion of said second conductive path of said element, said windings being connected together in reverse sense and coupled to said second source of current so as to produce a magnetic field in said element, the magnetic saturation of said element produced by said windings being suificient to produce a significant change in the skin-effect resistance of said element at the frequency of said first source.

2. Apparatus for modulating high 'frequency current comprising a source of high frequency current, a conductive control element having input and output terminals and first and second separate conductive paths connecting said input and output terminals, said element being coupled to said source and arranged to carry high frequency current between said input and output terminals, a source of modulating signals and bias current, a first winding surrounding at least a portion of said first conductive path of said element, a second winding surrounding at least a portion of said second conductive path of said element, said windings being substantially identical and connected together in reverse sense to produce a magnetic field in said element, and means coupling said windings to said source of modulating current, the variations in the magnetic saturation of said element produced by the fiow of said modulating current through said windings being sufiicient to produce a significant variation in the skin-efiect resistance of said element at the frequency of said first source.

3. Apparatus for modulating high frequency current comprising: a first source of high frequency current, a conductive element having input and output terminals and first and second conductive paths connected between said input and output terminals, said element being coupled to said source and arranged to carry high frequency current between said input and output terminals, a source of modulating signals, a control winding coupled to said modulating signal source and arranged to control the magnetic saturation of said conductive element, said winding having a first winding portion surrounding at least a portion of the first conductive path of said element, said winding having a second winding portion surrounding at least a portion of the second conductive path of said element, said second winding portion being wound in the c pposite direction from said first winding portion and being connected in series therewith, whereby the electrostatic and electromagnetic coupling between the element and the control winding are cancelled out, the variations in the magnetic saturation of said element produced by the flow of said modulating current through said windings being sufiicient to produce a significant variation in the skin efiect resistance of said element at the frequency of said first source.

4. Apparatus for modulating high frequency current comprising: a first source of high frequency current, a conductive ferromagnetic element having input and output terminals and first and second conductive paths each connected between said input and output terminals and forming a closed loop magnetic circuit passing through each of said paths, said element being coupled to said source and arranged to carry high frequency current to be modulated between said input and output terminals, a source of modulating signals, a control winding coupled to said modulating signal source and arranged to control the magnetic flux saturation of said conductive element, said winding having a first winding portion surrounding at least a portion of the first conductive path of said element, said winding having a second winding portion surrounding at least a portion of the second conductive path of said element, said second winding portion being wound in the opposite direction from said first winding portion and being connected in Series opposition therewith, the magnetic flux in said element induced by said control winding traveling around said loop magnetic circuit through each of said conductive paths so that the electrostatic and electromagnetic coupling between the element and the control Winding are cancelled out, the variations in the magnetic saturation of said element produced by the flow of said modulating current through said control Winding being suflicient to produce a significant variation in the skin effect resistance of said element at the fre quency of said first source.

References Cited in the file of this patent UNITED STATES PATENTS 

